Technical Abstract:
Fusarium species that cause maize ear rot and wheat head blight produce trichothecenes which can enhance virulence on some plant hosts. Cochliobolus carbonum, which causes leaf head blight of maize, produces HC-toxin, which is essential for virulence. In both of these pathogens, genes for toxin efflux pumps are closely linked to genes for toxin biosynthesis. TOXA from C. carbonum and Tri12 from F. sporotrichioides are members of the multifacilitator superfamily (MFS) of multidrug efflux pumps. Efflux pumps have been proposed to play a role in protecting fungi from their own toxic metabolites. Gene disruption of Tri12 in F. sporotrichioides has yielded strains that are more than 95% inhibited in trichothecene biosynthesis, but that are only slightly inhibited in growth rate even in the presence of added trichothecenes. These data indicate that Tri12 plays an important role in trichothecene biosynthesis but is not essential for trichothecene resistance. In contrast, all attempts to disrupt both copies of the TOXA gene in C. carbonum have been unsuccessful, suggesting that TOXA is essential for growth in toxin-producing strains. Experiments are in progress to identify a Tri12 homologue in F. graminearum which causes maize ear rot and head blight of wheat and other small grains. Field tests have shown that trichothecenes increase virulence of F. graminearum on maize and wheat. Therefore, Tri12, TOXA, and other toxin efflux pumps may have potential as transgenes to enhance resistance to toxins and to disease in cereal crops.